How to insulate floor joists with fiberglass
Hey All,
I am building an addition onto a pole barn in climate zone four (KY). I need to insulate the crawl space beneath the addition. Here are the details:
– At its most shallow the crawl space will be 1′ deep. It won’t be possible to install anything from below once the floor is framed.
– The addition is built on piers and spans over a slope. Sealing/insulating at the walls to control vapor is not an option.
I would like to use fiberglass insulation because of how much cheaper it is than rigid insul., but I have concerns about moisture and rodents.
My thought would be I could tack furring strips to the joists
Then install house wrap on 7/16 OSB and rest it on the furring strips. The WRB could be sealed to the joists using a window and door caulk.
Would this allow moisture to get out properly?
Is caulk the best way to seal the WRB to the joist, is there a tape that would work long term to seal the wrap to the joists?
Will the wrap hold up long term?
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Replies
Nick,
Can you clarify: Are you building a crawlspace, or a building on piers which will be open on all sides?
The building is going to be built on piers and open on all sides.
Nick,
Thanks.
It's a lot better floor if you can get a continuous layer of both rigid insulation under the joists to reduce thermal bridging, and either OSB or plywood to protect it. As you say it's hard to do that when you can't work from below, but there are two ways around that, and I've done both successfully:
- Build the floor in sections small enough to be flipped (say 8 ft x 12 ft). You add the foam board and plywood, making sure to leave strips of solid lumber where it bears, then turn it over and secure it to the beams. You can then fill the cavity with batts and add the sub-floor.
- Frame a floor of 2"x4"s at 24"oc, sheath the top with OSB or plywood, cover with a layer of foam board, then build the load-bearing floor on top. The link shows a drawing of this approach: https://www.southmountain.com/wp-content/uploads/2019/02/19-Small-Home-Harmony-Article-PDF.pdf
In either case, if the floor is very close to grade, you may want to use pt. plywood as the bottom layer. And if you have very aggressive pests you may want to cover it with hardware cloth.
That's very helpful! I've heard that it's important to have a way for any vapor/moisture that gets into an assembly to get out (i.e. don't put a vapor barrier on both sides of a wall system). Is this correct? With the second option you mentioned, the foam is a vapor barrier. Does 3/4" T&G offer enough permiability for the interstitial area to be ok?
Nick,
Yes, and the situation is different than walls or roofs for a couple of reasons.
- The stack effect works in your favour as it greatly reduces the chances of interior moisture moving down into the floor, and it moves any moist air in the floor system upwards towards your sub-floor, which is warmer, meaning the chances of it condescending there are much reduced. Like any assembly these floors benefit greatly from good air sealing.
Unlike walls or roofs there is no source of water infiltration from the outside. All you need to worry about is the moist air under the building. That gets stopped by the foam, but it is also good practice to put down a layer of poly (covered by gravel) on the ground to further limit the moisture load.